.....because of their ballistic transmission of electrons. But new research shows magnets can stop the migration of the electrons.

Rice physicist Junichiro Kono and his team had been studying the Aharonov-Bohm effect; the interaction between electrically charged particles and magnetic fields and how it relates to carbon nanotubes.  While doing so, they came to the unexpected conclusion that magnetic fields can turn highly conductive nanotubes into semiconductors. [Kono et al., Phys. Rev. Lett. (2010), 105, 017403].

"When you apply a magnetic field, a band gap opens up and it becomes an insulator," said Kono, a Rice professor in electrical and computer engineering and in physics and astronomy. "You are changing a conductor into a semiconductor, and you can switch between the two. So this experiment explores both an important aspect of the results of the Aharonov-Bohm effect and the novel magnetic properties of carbon nanotubes."
The author of the papers successfully measured the magnetic susceptibility of a variety of nanotubes for the first time; they confirmed that metallics are far more susceptible to magnetic fields than semiconducting nanotubes, depending upon the orientation and strength of the field.
Single-walled nanotubes (SWNTs), rolled-up sheets of graphene would all look the same to the naked eye if one could see them. But a closer look reveals nanotubes come in many forms, or chiralities, depending on how they're rolled. Some are semiconducting; some are highly conductive metallics. The armchair nanotube, so-called because the open ends form a pattern that looks like armchairs have conductive properties.
Kono and Searles traveled to the Tsukuba Magnet Laboratory at the National Institute for Materials Science (NIMS) in Japan, where the world's second-largest electromagnet was used to tease a refined ensemble of 10 chiralities of SWNTs, some metallic and some semiconducting, into revealing some of their properties.
By building the power of the magnet up to 35 tesla they found that the nanotubes would begin to align themselves in parallel and that the metallics reacted far more strongly than the semiconductors. Spectroscopic analysis confirmed the metallics, particularly armchair nanotubes, were two to four times more susceptible to the magnetic field than semiconductors and that each chirality reacted differently.
Kono said the work would continue on purified batches of nanotubes. That should yield more specific information about their susceptibility to magnetic fields, though he suspects the effect should be even stronger in longer metallics. "This work clearly shows that metallic tubes and semiconducting tubes are different, but now that we have metallic-enriched samples, we can compare different chiralities within the metallic family," he said.